Wash the zucchini and peppers. Core the peppers, clean the mushrooms and cut the vegetables into bite-sized pieces. Peel the onion and cut into rings.
Mix a marinade out of the oil and tomato ketchup, add the onion rings and pour everything over the zucchini, pepper and mushroom pieces. Stir gently and cover and marinate in the refrigerator for a good four hours. Stir at least once in between.
Prepare four large sheets of aluminum foil, distribute the slightly drained vegetables on top, pour a little salt and pepper over the vegetables and top each packet with 10 g herb butter, then turn the foil upwards.
Place the vegetable parcels on the grill. It takes about 15 minutes for the vegetables to be cooked through.